1. Field of the Invention
The present invention relates to a dummy wafer used in the process for manufacturing semiconductor devices such as a semiconductor memory and a semiconductor integrated circuit.
2. Description of the Related Art
In manufacturing a semiconductor device, a silicon wafer polished to a mirror surface is used as a substrate, and a polysilicon or Si.sub.3 N.sub.4 film is formed on the substrate by CVD or a W thin film is formed on the substrate by sputtering according to the type of the intended semiconductor device.
Quality requirements keep becoming restrict, and very high levels of processing accuracy and cleanliness are required as the performance and integration density of a semiconductor device are increased. Therefore, in the line for manufacturing a semiconductor device, a silicon wafer which is polished to a mirror surface in the same manner as the product is used as a test wafer (referred to as a "idummy wafer" hereinafter) for checking various inspection items so that each processing is performed while confirming processing accuracy and cleanliness in each process. Examples of such a dummy wafer include a dummy wafer used for monitoring the film thickness after film formation, a dummy wafer for checking cleanliness in each process by counting particles which adhere to a wafer, and the like.
Since the line for manufacturing a semiconductor device generally comprises at least 100 processes, many dummy wafers are required. A silicon wafer is expensive and is thus desired to be repeatedly used as the dummy wafer as many times as possible. For example, the dummy wafer used for monitoring the film thickness is reused after the thin film formed thereon is removed by etching. However, the etched silicon wafer has poor surface properties, and must be thus polished to a mirror surface again. In addition, since the properties of the silicon wafer itself depend upon the material used for film formation, it is necessary to remove the deteriorated layer by etching. In such a case, even if a mirror surface is obtained by re-polishing the silicon wafer after the etching, since the thickness of the wafer is gradually decreased, the number of times of repeated use is limited.
Examination is thus made on a substitute which exhibits the same characteristics as those of the silicon wafer and excellent etching resistance when used as the dummy wafer.
Examples of substitutes for t he silicon wafer include dummy wafers comprising ceramic substrates of SiC, Si.sub.3 N.sub.4 and the like, which are polished to a mirror surface. However, such dummy wafers are inferior to the silicon wafer from the viewpoints of mirror surface properties and evenness, and has the problem of producing dust particles in the manufacturing process and thus causing a contamination source of a semiconductor device.